Fluted polymer boards are often formed in an in-line extrusion process. Raw polymer stock is melted in a feeder and forced through an extruder. The extruder forms material having a top sheet, a parallel bottom sheet, and a plurality of ribs that extend between the top and bottom sheets at spaced apart locations along the width of the material to define the flutes. One or both edge margins of the material can become deformed in the extrusion process. To exclude the deformed edge margins from the final board product, edge trimmers are arranged downstream of the extruder to separate the edge margins from the middle portion of the material. To limit polymer waste, some board forming processes return the trimmed edge margins to the feeder for recycling.